Software Engineering - Ch8

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 1
System models

Objectives
● To explain why the context of a system
should be modelled as part of the RE
process
● To describe behavioural modelling, data
modelling and object modelling
● To introduce some of the notations used in
the Unified Modeling Language (UML)
● To show how CASE workbenches support
system modelling

Topics covered
● Context models
● Behavioural models
● Data models
● Object models
● CASE workbenches

System modelling
● System modelling helps the analyst to understand
the functionality of the system and models are used
to communicate with customers.
● Different models present the system from different
perspectives
• External perspective showing the system’s context or
environment;
• Behavioural perspective showing the behaviour of the
system;
• Structural perspective showing the system or data
architecture.

Model types
● Data processing model showing how the data is
processed at different stages.
● Composition model showing how entities are
composed of other entities.
● Architectural model showing principal sub-systems.
● Classification model showing how entities have
common characteristics.
● Stimulus/response model showing the system’s
reaction to events.

Context models
● Context models are used to illustrate the
operational context of a system - they show
what lies outside the system boundaries.
● Social and organisational concerns may
affect the decision on where to position
system boundaries.
● Architectural models show the system and its
relationship with other systems.

The context of an ATM system

Process models
● Process models show the overall process
and the processes that are supported by the
system.
● Data flow models may be used to show the
processes and the flow of information from
one process to another.

Equipment procurement process

Behavioural models
● Behavioural models are used to describe the
overall behaviour of a system.
● Two types of behavioural model are:
• Data processing models that show how data is
processed as it moves through the system;
• State machine models that show the systems
response to events.
● These models show different perspectives so
both of them are required to describe the
system’s behaviour.

Data-processing models
● Data flow diagrams (DFDs) may be used to
model the system’s data processing.
● These show the processing steps as data
flows through a system.
● DFDs are an intrinsic part of many analysis
methods.
● Simple and intuitive notation that customers
can understand.
● Show end-to-end processing of data.

Order processing DFD

Data flow diagrams
● DFDs model the system from a functional
perspective.
● Tracking and documenting how the data
associated with a process is helpful to
develop an overall understanding of the
system.
● Data flow diagrams may also be used in
showing the data exchange between a
system and other systems in its environment.

Insulin pump DFD

State machine models
● These model the behaviour of the system in
response to external and internal events.
● They show the system’s responses to stimuli so are
often used for modelling real-time systems.
● State machine models show system states as nodes
and events as arcs between these nodes. When an
event occurs, the system moves from one state to
another.
● Statecharts are an integral part of the UML and are
used to represent state machine models.

Statecharts
● Allow the decomposition of a model into sub-
models (see following slide).
● A brief description of the actions is included
following the ‘do’ in each state.
● Can be complemented by tables describing
the states and the stimuli.

Microwave oven model

Microwave oven state description
State Description
Waiting The oven is waiting for input. The display shows the current time.
Half power The oven power is set to 300 watts. The display shows ‘Half power’.
Full power The oven power is set to 600 watts. The display shows ‘Full power’.
Set time The cooking time is s et to the user’s input value. The display shows the cooking time
selected and is updated as the time is set.
Disabled Oven operation is disabled for safety. Interior oven light is on. Display shows ‘Not
ready’.
Enabled Oven operation is enabled. Interior oven light is off. Display shows ‘Ready to cook’.
Operation Oven in operation. Interior oven light is on. Display shows the timer countdown. On
completion of cooking, the buzzer is sounded for 5 s econds. Oven light is on. Display
shows ‘Cooking complete’ while buzzer is sounding.

Microwave oven stimuli
Stimulus Description
Half power The user has pressed the half power button
Full power The user has pressed the full power button
Timer The user has pressed one of the timer buttons
Number The user has pressed a numeric key
Door open The oven door switch is not closed
Door closed The oven door switch is closed
Start The user has pressed the start button
Cancel The user has pressed the cancel button

Microwave oven operation

Semantic data models
● Used to describe the logical structure of data
processed by the system.
● An entity-relation-attribute model sets out the
entities in the system, the relationships between
these entities and the entity attributes
● Widely used in database design. Can readily be
implemented using relational databases.
● No specific notation provided in the UML but objects
and associations can be used.

Library semantic model

Data dictionaries
● Data dictionaries are lists of all of the names used in
the system models. Descriptions of the entities,
relationships and attributes are also included.
● Advantages
• Support name management and avoid duplication;
• Store of organisational knowledge linking analysis, design
and implementation;
● Many CASE workbenches support data dictionaries.

Data dictionary entries
Name Description Type Date
Article
Details of the published article that may be ordered by
people using LIBSYS.
Entity 30.12.2002
authors
The names of the authors of the article who may be due
a share of the fee.
Attribute 30.12.2002
Buyer
The person or organisation that orders a co py of the
article.
Entity 30.12.2002
fee-
payable-to
A 1:1 relationship between Article and the Copyright
Agency who should be paid the copyright fee.
Relation 29.12.2002
Address
(Buyer)
The address of the buyer. This is used to any paper
billing information that is required.
Attribute 31.12.2002

Object models
● Object models describe the system in terms of
object classes and their associations.
● An object class is an abstraction over a set of
objects with common attributes and the services
(operations) provided by each object.
● Various object models may be produced
• Inheritance models;
• Aggregation models;
• Interaction models.

Object models
● Natural ways of reflecting the real-world
entities manipulated by the system
● More abstract entities are more difficult to
model using this approach
● Object class identification is recognised as a
difficult process requiring a deep
understanding of the application domain
● Object classes reflecting domain entities are
reusable across systems

Inheritance models
● Organise the domain object classes into a hierarchy.
● Classes at the top of the hierarchy reflect the
common features of all classes.
● Object classes inherit their attributes and services
from one or more super-classes. these may then be
specialised as necessary.
● Class hierarchy design can be a difficult process if
duplication in different branches is to be avoided.

Object models and the UML
● The UML is a standard representation devised by
the developers of widely used object-oriented
analysis and design methods.
● It has become an effective standard for object-
oriented modelling.
● Notation
• Object classes are rectangles with the name at the top,
attributes in the middle section and operations in the
bottom section;
• Relationships between object classes (known as
associations) are shown as lines linking objects;
• Inheritance is referred to as generalisation and is shown
‘upwards’ rather than ‘downwards’ in a hierarchy.

Library class hierarchy

User class hierarchy

Multiple inheritance
● Rather than inheriting the attributes and services
from a single parent class, a system which supports
multiple inheritance allows object classes to inherit
from several super-classes.
● This can lead to semantic conflicts where
attributes/services with the same name in different
super-classes have different semantics.
● Multiple inheritance makes class hierarchy
reorganisation more complex.

Multiple inheritance

Object aggregation
● An aggregation model shows how classes
that are collections are composed of other
classes.
● Aggregation models are similar to the part-of
relationship in semantic data models.

Object aggregation

Object behaviour modelling
● A behavioural model shows the interactions
between objects to produce some particular
system behaviour that is specified as a use-
case.
● Sequence diagrams (or collaboration
diagrams) in the UML are used to model
interaction between objects.

Issue of electronic items

Structured methods
● Structured methods incorporate system
modelling as an inherent part of the method.
● Methods define a set of models, a process
for deriving these models and rules and
guidelines that should apply to the models.
● CASE tools support system modelling as
part of a structured method.

Method weaknesses
● They do not model non-functional system
requirements.
● They do not usually include information
about whether a method is appropriate for a
given problem.
● The may produce too much documentation.
● The system models are sometimes too
detailed and difficult for users to understand.

CASE workbenches
● A coherent set of tools that is designed to
support related software process activities
such as analysis, design or testing.
● Analysis and design workbenches support
system modelling during both requirements
engineering and system design.
● These workbenches may support a specific
design method or may provide support for a
creating several different types of system
model.

An analysis and design workbench

Analysis workbench components
● Diagram editors
● Model analysis and checking tools
● Repository and associated query language
● Data dictionary
● Report definition and generation tools
● Forms definition tools
● Import/export translators
● Code generation tools

Key points
● A model is an abstract system view.
Complementary types of model provide
different system information.
● Context models show the position of a
system in its environment with other systems
and processes.
● Data flow models may be used to model the
data processing in a system.
● State machine models model the system’s
behaviour in response to internal or external
events

Key points
● Semantic data models describe the logical
structure of data which is imported to or
exported by the systems.
● Object models describe logical system
entities, their classification and aggregation.
● Sequence models show the interactions
between actors and the system objects that
they use.
● Structured methods provide a framework for
developing system models.

Software Engineering - Ch8

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Software Engineering - Ch8